Ignore ruff cache (#19854)

When I run `ruff` locally, either `ruff` or one of the linting scripts
adds `.ruff_cache` to the `.gitignore` file. This PR adds that line so
that running linters doesn't result in a dirty git working tree (and to
ignore ruff's cache, of course).

### Pull Request Checklist

<!-- Please read
https://element-hq.github.io/synapse/latest/development/contributing_guide.html
before submitting your pull request -->

* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
  - Use markdown where necessary, mostly for `code blocks`.
  - End with either a period (.) or an exclamation mark (!).
  - Start with a capital letter.
- Feel free to credit yourself, by adding a sentence "Contributed by
@github_username." or "Contributed by [Your Name]." to the end of the
entry.
* [x] [Code
style](https://element-hq.github.io/synapse/latest/code_style.html) is
correct (run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

.gitignore

@@ -53,6 +53,7 @@ __pycache__/
 !/.coveragerc
 /.coverage*
 /.mypy_cache/
+/.ruff_cache/
 /.tox
 /.tox-pg-container
 /build/

changelog.d/19854.misc

@@ -0,0 +1 @@
+Add `.ruff_cache/` directory to `.gitignore`.